Heat absorber

[AC1985]

Hi,

I am trying to design an exhaust system that would make it very hard for a heat-seeking missile to identify.

I have read that the F-117 stealth fighter uses "heat conducting materials" in its exhaust ducts to take some heat away from the gasses leaving the engine.

I am thinking about surrounding the exhaust with the foam that the space shuttle uses to protect itself from the high temperatures experienced during re-entry to the earth's atmosphere. This material can hold a MASSIVE amount of heat energy.

However, my thermodynamics is a little rusty, since I haven't studied it for a couple of years.

I know that the zeroth law of thermodynamics says if the temperature of two bodies in contact is the same, there is no flow of energy between them.

I know that if the exhaust gas was held in a container, over time it would reduce its temperature until it was equal to the temperature of the ambient atmosphere.

The problem is that the exhaust gas is constantly being renewed, so the temperature of the gas at the point where it leaves the turbine is always the same (about 720 decgrees C). If the space shuttle foam was next to the gas flow, I think it would just heat up until it is also at 720 degrees C, and the exhaust gas would not be able to cool down.

Am I right or wrong?

I would think that two bodies would be in equilibrium when they both have the same internal energy, so if the absorbing material had a low specific heat capacity then it could maintain a higher temperature than the exhaust gas, allowing some reduction in exhaust temperature for a sustained period.

 

[MintJulep]

Are you refering to the shuttle's heat tiles. They are ceramic of some sort, not foam. Also, I don't think they absorb heat, I think they were chosen for their ability to insulate and not absorb heat under the harsh conditions of re-entry.

Should be lots of info available on them.

To answer your question (sort of):

If you insulate the exhaust ducting then the exhaust gas stream will not cool down as it passes through the duct. Therefore the only thing that can happen is that it will be hotter when it leaves the duct than it would be if the duct were uninsulated.

 

[dcasto]

I believe the exhaust is cooled by blending a lot of cold air passing around the plane. If the F-117 were on the ground with engines running, it would be a sitting duck for a heat seaking system.

 

[IRstuff]

Nothing short of a large body of water can absorb the heat from a turbine engine.

B2 and F117 and F22 use mixing to cool the exhausts and hid them on the tops of the wings.

That's the only gig in town; mix and spread.

One thing that's rather funny is that the RAM coating on the fuselages are VERY good IR absorbers and emitters.

TTFN

 

[AC1985]

Thanks guys.

I am aware that mixing with cool air is a good way to cool down the exhauust gas, And I guess if I used a thin but wide rectangular exhaust I can get a lot of mixing.

Its just that on wikipedia it says that the F-117 uses heat absorbing materials in its ducts to cool down the exhaust gases and I was trying to think of a way that this could be possible.

I suppose that contacting a mass with a large specific heat capacity would allow a large amount of energy to be absorbed, but this conduction would be likely to be pretty slow, too slow to cool the exhaust gas enough to have a good effect.

Thinking Fourier's Law of conduction (dQ/dt=kA.dT) to find the heat energy flow, I would need a material with a high value of thermal conductivity.
If the specific heat capacity was very large, the increase in temperature would be low compared to the amount of heat energy being transfered to the heat sink (Q=mc.dT). When the temperature of the heat sink is less than the gas, the temperature of the gas will decrease.

Does anybody know of a material with high thermal conductivity and specific heat capacity?

 

[IRstuff]

No mention of what you're talking about:

http://en.wikipedia.org/wiki/Stealth_technology

http://www.aeronautics.ru/f117a.htm

http://en.wikipedia.org/wiki/F-117

I suggest that you do the math on the amount of heat generated by a jet turbine and compare that to the specific heat of available materials.

TTFN

 

[25362]

Just for the record, the zeroth law says that if systems A and B are each in thermodynamic equilibrium with C, then A and B are in thermodynamic equilibrium with each other.

 

[dcasto]

Think about it, if a material absorbs heat, it gets hot and will show up on a heat monitor. What there is is a lot of surface area to move te heat away and mix it with air over a larger area. Its like the mesh they put in a fuel tank. If the fuel tank is hit with a shell, the heat from burning fuel is spread out and cooled so the won't be an explosion. We use a similar system as a "flame aresstor" on the air intake to furnaces in the oil industry.

 

[willard3]

snip
Its just that on wikipedia it says that the F-117 uses heat absorbing materials in its ducts to cool down the exhaust gases and I was trying to think of a way that this could be possible.
snip

Wikipedia is unvetted information and, as such, inaccurate.

Words will hold still for anyone.........

 

[JKEngineer]

I think that the wikipedia article (from what you have said, not from reading it) indicates that the materials surrounding the exhaust are conductive -- so that they take heat away ("absorb") from the gas and deliver it elsewhere -- not hold it.

Thermal equilibrium is based on temperature, not internal energy.

The mix and spread description above is valid and is potentially augmented by conducting heat out of the gas to other places that then reject it to the surroundings.

I agree with IRStuff's suggestion that you calculate the heat absorption capacity of the proposed solids. Don't forget that you are taking a significant fraction of the energy content of the burned fuel and proposing applying it to solids in the plane. Sort of like a flying foundry. ;-)

Jack

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com